Although the design of prior art chairs has considered the requirement of ergonomics, since the chair are mass-produced by a standard specification, the user must make his body suit the design of the chair. However, since everybody has his individual physique, thus the design of ergonomics can not be sufficient enjoyed by the user. Often as the user sits by the back to against the chair back, he will feel ache on his back since generally a space is formed between the user's back and the back pad of chair. Therefore, there is an eager demand to have a novel design of back pad of a chair which can be adjusted according to the requirement of users.